Apparatus for reconditioning tubes operating with townsend avalanche



Apnl 14, 1970 L. F.. GILBERT 3,506,331

I APPARATU$"FOR ,RECONDITIONING TUBES OPERATING WITH TOWNSEND AVALANCHEFiled Dec. 29. 1966 I a l I I I I l l I l I I 1 I I I I I I I I I I I II 1 1- I I I 1 .J

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I I I I l I I I l I I I INVENTOR. LYMAN E GILBERT United States Patent MU.S. Cl. 25083.6 Claims ABSTRACT OF THE DISCLOSURE A system and processfor restoring to its original work function .an electron tube thatoperates with a townsend avalanche and which has had its work functionaltered because of electrode contamination. In the system and method theelectrodes are heated for a short time by passing a current through thetube to raise the electrode temperature to a value such that thecontaminates will be evaporated from the electrode surface and willcondense on the envelope of the tube.

SUMMARY OF THE INVENTION Electron tubes that operate with a townsendavalanche are designed originally with a particular work function suchthat they respond to a particular energy spectrum. Illustrative of sucha tube is the glow discharge tube that may be employed in flame scannersystems with the tube having an energy sensitive spectrum range of 2000*to 3300 angstroms such that it responds only to energy emitted by theflame and not to energy emitted by glowing refractory, slag or the like.During the operation of such tubes it has been found that contaminatesgradually deposit on the electrodes (one or both of them) so that thecharacteristics of the tube change with the tube eventually being nolonger suitable for its intended operation. These contaminates may beimpurities from the glass or quartz envelope of the tube such as sodiumand silica compounds or they may be impurities from the tungsten orother electrodes themselves such as copper or tungsten oxide impuritieswhich may come from the inner portion of the electrode to the surfacethereof. These contaminates will change the work function of the tube sothat the energy spectrum to which the tube responds will be changed.Thus in the case of the glow discharge tube of the flame scanner thetube will begin to respond to energy sources other than the flame and beless sensitive to the flame energy source.

It has been found that this ditficulty can be overcome and the tubereconditioned so that its Work function is returned generally to theoriginal value by heating the electrodes of the tube to drive thecontaminates from the electrode surface and deposit them on the tubeenvelope.

This is preferably accomplished with the tube in situ i.e., Within thecircuit in which it is intended to operate, for exarrfple in the flamescanner circuit. The heating is accomplished by passing a currentthrough the tube that is considerably greater than that ordinarilypassed through it with this current being maintained only for a veryshort time such as one to ten seconds. This heats the electrodes withoutsubstantially heating the tube envelope and so that the temperature ofthe electrodes is elevated to a high value sufficient to evaporate thecontaminates from the electrode surface. The tube envelope is maintainedat a sufliciently low value so that these contaminates will condense onthe surface of the envelope. The circuit in which the tube is connectedis preferably provided with an automatic control device to periodicallyeffect this heating and accordingly decontamination of the electrodes3,506,831 Patented Apr. 14, 1970 ICC such that the tube is maintainedgenerally at its original work function.

DESCRIPTION OF THE DRAWING The single drawing figure is in the nature ofa circuit diagram showing one environment in which the present inventionmay be utilized.

DETAILED DESCRIPTION Referring to the drawing, there is illustratedtherein a circuit for a flame scanner which is generally similar to thatof my U.S. Patent 3,286,093 of Nov. 15, 1966, with the exception in theillustrative arrangement the tube that operates with townsend avalanchehas pulsating DC rather than AC applied to it and has a current limitingresistor connected across the tube electrodes. (Reference to said patentis made for details of the theory of operation.) In this circuit thetube 10 operating with townsend avalanche is a glow discharge tubehaving spaced electrodes 36 and 37 and receiving its electric potentialfrom the source 12 by means of the transformer 14. This transformer 14forms part of the electric circuit into which the tube is connected andthis circuit includes the diode 16 connected to one electrode of thetube through the fuse 18 with the other electrode of the tube beingconnected with the other terminal of the secondary of transformer 14.This connection includes, resistor 20, capacitor 22 and resistor 24. Thestep-up transformer 14 is thus effective to apply a DC supply to thetube 10 and which, for example, may have a value of between 300 and 750volts.

The glow discharge tube 10 preferably has a work function such that itresponds to an energy spectrum range of between 2000 and 3300 angstroms.In operation the tube is located such that it directly views the flamethat is to be under surveillance. Energy from the flame thus passesthrough the quartz envelope of the tube so that a photon of properenergy content and proper wave length, will trigger a townsend avalancheof the tube. The portion of the circuit designated generally 26 may betermed a transmitter and is effective to produce a pulse signal uponavalanche occurring in the tube. This signal is separated from thepotential source supplied to the tube with resistor 24 and capacitor 28comprising the signal separator network. The capacitor 22 and resistor20 are for the purpose of increasing the time duration of the shortpulse produced at avalanche.

The pulse signals thus produced and separated by the transmitter arecoupled into the receiver circuit 34, which is identical to that ofpreviously identified Patent 3,286,- 093, to produce an output signal asa result of the device detecting the presence of a flame.

During the operation of the tube 10 it has been found that one or bothof the electrodes progressively become contaminated by impuritiesdepositing on the surfaces thereof (one when DC is applied to the tubeand both when AC is applied to the tube). These contaminates may comefrom the surface of the envelope of the tube, for example, they may besodium or silica compounds found in the quartz or glass, and they mayalso be impurities found in the tungsten which work to the surface ofthe electrode. In the illustrative example the tube may be provided witha pair of spaced tungsten electrodes in an atmosphere of hydrogen. Inthe the manufacture of the tube particular efforts is may to eliminateimpurities. However, notwithstanding such efforts all impurities cannotbe removed and those that remain gradually deposit on the electrodesduring operation of the tube. As these deposits increase, the workfunction of the tube progressively changes such that it no longerresponds to or is no longer restricted to the original spectral responseand therefore the tube becomes ineffective for its intended purpose.

In order to prevent this from happening there is provided, in theillustrative circuit, means for periodically heating the electrodes ofthe tube in a manner which will remove these contaminates. This isachieved by connecting electrode 36 to the power supply throughresistance 40 and switch 42. Thus in closing the switch 42 tube isconnected across the secondary of the transformer with only fuse 18 andresistance 40 being in series with the tube. The purpose of resistance40 is to sufficiently limit the current so as to not blow the fuse 18.The resistance 40 may thus be eliminated and in its stead a bypassprovided around the fuse 18 with this bypass being completedsimultaneously with the closing of the switch 42.

The power supply for the tube 10 is a partially filtered DC by means ofthe capacitor 33. However, even if the power supply were an unfilteredDC (pulsating 60 c.p.s.), tube 10, once triggered, would continue toconduct with switch 42 closed when the voltage dropped to zero becausethe heavy current prevents the recovery of the tube. The current flowthrough the tube determines the degree of ionization of the gasestherein and, therefore, markedly affects the recovery time of the tube.Thus once the tube is triggered into heavy conduction it will continueconducting while switch 42 is closed. Therefore the time of conductioncan be controlled by switch 42.

Switch 42 is a time operated switch such that it closes periodically fora short time depending upon the characteristics of the tube and thecircuit into which it is connected as well as upon the operatingconditions to which the tube is subjected. The depositing ofcontaminates upon the electrodes of the tubes is influenced by thetemperature at which the tube operates with the contamination beingaccelerated at higher temperatures. The time requirement for removal ofthe contaminates is dependent upon the temperature to which theelectrodes are heated, with the higher the temperature the faster willbe the removal. Illustrative of the time required is between 1 and 10seconds and illustrative of the temperature to Which the electrodes maybe heated is between 1400 and 4500 F., with tungsten electrodes. Thetemperature rise of the electrodes must be sufliciently rapid so thatthe electrodes are heated to their desired temperature where thecontaminates will evaporate from the electrodes while the envelope ofthe tube is not substantially increased in temperature but remains at atemperature below the condensation temperature of the vaporizedcontaminates. Thus, the contaminates vaporize from the electrodes andcondense upon the inner surface of the envelope whereby the electrodesare returned to their pure state that they originally had and the workfunction of the tube is that originally provided. It will be understoodthat in this process of removing contaminate from the electrodes thatthey must not be elevated to such a high temperature as to damage theelectrodes. Thus in the case of tungsten the temperature should not beabove 4500 F.

It is desirable and extremely convenient to provide an automatic meanswithin the circuit that utilizes the avalanche-operating tube toperiodically raise the temperature of the electrodes as previouslydescribed to remove contaminates from them. In the illustrative flamescanner circuit in one environment, it has been found that if switch 42is closed for approximately 7 seconds each hour during the period thatthe tube 10 is viewing a flame, the work function of the tube remainsgenerally at its original and desired value and thus the electrodes ofthe tube are maintained generally free of contaminates.

The improved process of the invention can, if desired, be carried out onthe bench and need not be carried out in the circuit in which the tubeis employed. The tube may be connected into a separate and specialcircuit for reconditioning and an artificial source of excitation usedto provide the energy for triggering the tube into avalanche, such as anultra-violet lamp in the case of the 2000-3000 angstrom tube previouslymentioned. In this separate circuit a sufficient voltage may be appliedto the tube for a suflicient length of time such that the currentpassing through the tube will heat the electrodes as previouslydescribed and thus evaporate from the electrodes surface thecontaminates deposited thereon.

The voltage developed across the secondary 14 of the transformer may beapproximately 700 volts and as previously mentioned switch 42 may closefor approximately 7 seconds once each hour, the current then passingthrough tube 10 at the closing of switch 42 is between 400 and 600 ma.and the electrodes of the tube are heated to a dull red during thistime.

Accordingly, it will be appreciated that a novel system and process hasbeen devised whereby tubes, such as Geiger-Mueller tubes and glowdischarge tubes, operating with a townsend avalanche, have contaminatesremoved from the tube electrodes so as to maintain the tube at or returnthe tube to its desired work function.

It is to be understood that the invention is not limited to the specificembodiment herein illustrated and described but may be used in otherways without departure from its spirit and that various changes can bemade which would come within the scope of the invention which is limitedonly by the appended claims.

I claim: 4

1. In an electrical circuit utilizing a glow discharge tube subjected toenergy to periodically fire said tube and combination of means toperiodically connect the tube across a potential and for a period oftime such that upon firing the tube a current will pass through theelectrodes thereof of a magnitude to heat the same to a sufficientlyhigh value to evaporate the contaminates from the surface thereof withthe environment in which the tube is operating being such that theenvelope is maintained at a temperature below the condensationtemperature of these contaminates whereby the contaminates are condensedupon the envelope surface.

2. A flame detector including an electric circuit having a tuberesponding to energy of a flame to cause a townsend avalanche to occurin the tube thereby producing a pulse signal, means effectivelyresponsive to this pulse signal to produce an output signal, saidcircuit including a source of potential across which the tube isconnected with there being means in the circuit to limit the currentflow therethrough, and means effective to connect said tube across saidpotential in such a manner that upon firing of the tube a current flowmuch greater than normal is obtained through the tube electrodes so asto heat the same to a sufficiently high value to evaporate contaminatesfrom the surface thereof.

3. The organization of claim 2 including a time operated switch means inbypass relation with at least some of said current limiting means insaid circuit.

4. A flame scanner circuit comprising in combination a glow dischargetube adapted to be subjected to energy from a flame for triggering thetube into avalanche, an electric circuit into which said tube isconnected and including a source of potential, means providing an outputsignal developed incident to said tube being subjected to energy of aflame, and means operative to automatically periodically heat saidelectrodes to a sufficiently high temperature to evaporate contaminatestherefrom comprising a time operated switch means for connecting saidtube across said potential.

5. A flame detector comprising in combination an electric circuitincluding a source of potential and a tube operating with townsendavalanche and being fired upon receiving energy from a flame, with theavalanche of the tube producing a pulse signal in the circuit, saidcircuit including an impedance across which a voltage drop is developedby the current established at avalanche and means to periodically effectan increase in current through said tubes to heat the electrodes thereofto a sufliciently high value to evaporate contaminates therefrom, thislast 5 6 named means including a time operated switch means 3,103,5899/1963 Howling 313-93 X connected in parallel with said impedance.3,286,093 11/1966 Gilbert 250-83.3 X

References Cited JAMES W. LAWRENCE, Primary Examiner UNITED STATESPATENTS 5 C. R. CAMPBELL, Assistant Examiner 1,906,653 5/1933 Spencer316-26 2,746,831 8/1952 Chapman 31626 US 3,038,993 6/1962 Masuda 313-45X 25083.3; 3162, 26, 28

